This disclosure relates to methods for the manufacture of circuit laminates, and circuit boards containing such laminates.
Printed circuit boards having low dielectric constants and low dissipation factors are preferred for use with high frequency circuits. Printed circuit boards are typically manufactured from circuit laminates having a conductive layer, e.g., copper, fixedly attached to a dielectric substrate layer. Double clad circuit laminates have two conductive layers, one on each side of the dielectric substrate layer. Patterning a conductive layer of a laminate, for example by etching, provides a circuit. Multilayer circuits comprise a plurality of conductive layers, at least one of which contains a conductive wiring pattern.
For certain high frequency applications, foamed materials can be used as the dielectric substrate layer to provide the desirable dielectric and dissipation properties. Foamed circuit laminates are known, being described for example in U.S. Pat. No. 4,937,585 to Shoemaker et al. and U.S. Pat. No. 5,541,366 to Moaz et al.
The cellular structure of foams provide unique properties to dielectric substrates, but also give rise to processing difficulties, particularly when processed using conventional circuit board procedures and equipment. Use of open-celled foams is particularly problematic. For example, the foamed material can be subject to too much compression during lamination to form multi-layer circuit assemblies, which adversely affects the desired electrical properties of the multi-layered circuits. The foams are easily damaged by tooling holes and racks. Another processing problem associated with foamed dielectric substrates is the occurrence of wrinkling during lamination, which occurs as the foam material shrinks during cool down. This currently creates the need for a cumbersome pre-lamination stress relief process. It has further been found that foamed dielectric substrates can be susceptible to absorption of processing chemicals at the surface, edges, and through-hole walls of the foamed material.
There accordingly remains a need in the art for processes and materials for the manufacture of circuit laminates containing foamed dielectric substrates that are compatible with standard processing procedures and equipment.